Chemical pathways and kinetic rates of the N(4S) + N2 → N3 solid phase reaction: could the N3 radical be a temperature sensor of nitrogen ices in dense molecular clouds?

Abstract

Even though the N 3 radical has not yet been detected in the interstellar medium, its formation still remains a challenge. For a long time, bombardments of N 2 ices by energetic particles were the only way to from the azide radical as it was thought that ultraviolet (UV) photons were not strong enough to fragment the molecular nitrogen into N atoms. Consequently, it had been suggested that N 3 could be used as discriminator between ice radiolysis and ice photolysis until a very recent study that has shown that photodecomposition of molecular nitrogen by UV photons might also be a source of the azide radical. In contrast to all these nitrogen ice bombarding experiments, only two laboratory studies have investigated the N 3 formation where the reactants N and N 2 mixed in the gas phase were co-condensed at 12 K, and this raised a new question concerning whether N + N 2 → N 3 took place in the solid phase or in the gas phase. The experimental results of these two studies are contradictory and the problem of the characterization of N 3 formation by co-condensing atomic N and molecular N 2 has persisted to the present day. In this paper, we give a clear answer to this question, by investigating the kinetic rates of the N(4 S) + N 2 → N 3 reaction in the solid phase in the temperature range of 3-35 K. We find a rate constant of 7.7 × 10 −23 s −1 molecule −1 cm 3 for the azide radical formation in the solid phase and we provide new information on the N 3 infrared signature, which could be used to characterize the temperature and the structure of nitrogen ices.

VUV Photolysis of CH4–H2O mixture in methane-rich ices: Formation of large complex organic molecules in astronomical environments

“…As mentioned in the introduction part, many laboratory simulations of energetic decomposition of pure methane ice have been explored in order to underline the role of CH 4 fragmentation in the formation of complex organic molecules on the surfaces of icy bodies in the interstellar medium, comets and solar system objects. In this context, using infrared absorption signatures to indentify the photoproducts, Lin et al 4 16 . In the case of water, the bending mode of H 2 O at 1600 cm -1 has been used with the band strength of 1.2 10 -17 cm molecules -1 taken from Gerakines et al investigations 17 (the same value has been also predicted by DFT calculations for water monomer 18 ).…”

“…As mentioned earlier, the IR identification of CH 3 OH and CH 3 CH 2 OH has been based on previous studies [11][12][13][14][15][16][17][18][19][20][21][22][23] , however, propanol has been less studied and for this reason, we have investigated the behavior of the spectral position shifts of the C-O stretching vibration of CH 3 OH, CH 3 CH 2 OH and CH 3 CH 2 CH 2 OH in solid phase. In fact, as it will be discussed below, we have measured the C-O stretching peaks positions of solid CH 3 OH, CH 3 CH 2 OH, CH 3 CH 2 CH 2 OH at 1023.8, 1045.7, 1067.9 cm -1 , respectively.…”

Solid-state formation of CO and H2CO via the CHOCHO + H reaction

“…The amounts of the reactant consumed (CHOCHO) and products formed (CO and H 2 CO) have been evaluated from the IR spectra before and after H-bombardments of CHOCHO ice (figures 2a and 2b) and shown in table 3. The column densities N (molec cm -2 ) of the reactant and products are directly deduced from integrated intensities of the corresponding characteristic absorption bands (Mencos et al 2017). The column densities of glyoxal ice before and after H-bombardments have been calculated by probing the C=O stretching mode at 1709 cm -1 .…”

N2 Chemistry in Interstellar and Planetary Ices: Radiation-driven Oxidation

“…The preceding is based on reactions involving ions, but neutral free radicals also are important. As already stated, during an irradiation with MeV ions a track of excitations and ionizations will be produced by each ion as it travels through an ice, generating secondary electrons that produce their own trails of chemical change again to make the N 3 we observed, although more complex paths to N 3 exist (Mencos et al 2017). For N 2 -rich ices containing O 2 , excitation and dissociation of molecular oxygen will give O atoms from which nitrous oxide and ozone can form in reactions (11)-(13):…”